1. Field of the Invention
The present invention relates to sealing members for gas preheaters such as air preheaters, and sealing structures using such sealing members for the gas preheaters.
2. Related Art Statement
An air preheater is used for recovering heat from exhaust gases through heat exchange to preheat combustion air. Such preheaters are used in power generating boilers and ship boilers as well as chemical plants or equipment such as an oil-producing furnace, distillation furnace and a reforming furnace. FIG. 1 is a schematic perspective view of such an air preheater.
A rotor 11 is rotatably placed inside a rotary housing 17, and two radial sealing plates 13 are provided on each of side faces of the rotor 11. The rotor housing 17 is supported by a pedestal not shown, and the rotor is rotated in a direction of an arrow H.
A hot exhaust gas is passed through an upper side of the rotor 11 in a direction of an arrow D, whereas air is passed through a lower side of the rotor in the opposite direction as shown by an arrow F. An exhaust gas layer and an air layer are isolated from each other by the radial sealing plates 13. The rotor 11 is provided with heating elements, which absorb heat from the hot exhaust gas flowing in the D-direction. As the rotor 11 rotates, cold air passing along the heating elements is heated and sent to a boiler or the like in a direction of an arrow G. On the other hand, since heat is derived from the hot exhaust gas, cooled exhaust gas is discharged outside in a direction of an arrow E.
In such an air preheater, gaps exist between an outer peripheral portion of the rotor 11 and housing and between side faces of the rotor and the radial sealing plates 13, respectively, so that exhaust gas and air leak through these gaps which lowers thermal efficiency. Therefore, in order to improve sealing, it is important to make these gaps as small as possible.
Sealing of the gap between the outer periphery of the rotor 11 and the housing 17 is effected by a structural sealing in which the above gap between them is made as small as possible.
The temperature of the exhaust gas is in a range of 300.degree.-400.degree. C., and that of air is in a range of room temperature to about 100.degree. C. The rotor 11, which receives the heating elements, is deformed corresponding to such changes in temperatures. Accordingly, gaps due to expansion may form because of such changes in temperatures, resulting in deterioration of the sealing effect.
On the other hand, sealing of the gap between the side face of the rotor 11 and the radial sealing plate 13 is also effected by a structural sealing in which twelve or twenty diaphragm plates are provided on the side surface of the rotor, and a gap defined between the diaphragm plates and the radial sealing plate is made as small as possible.
Such structural sealings have a limited sealing effect. When the structural materials are corroded with a gas component in the exhaust gas, the gaps between the structural materials becomes greater, thereby lowering the sealing effect.
In view of this, it is expected that the sealing effect is improved by not only reducing the gaps between the structural materials but also, for example, sliding, under pressure, a sealing member attached to the outer peripheral portion of the rotor upon the opposite housing.
A slidable member to be used in a rotary type air preheater for combustion in a boiler in such as a thermal power generation plant is required to have excellent wear resistance, excellent corrosion resistance and excellent thermal shock resistance. Although a conventional sealing member uses an SS steel or a corrosion resistive steel, such a sealing member has inadequate wear resistance and corrosion resistance, so that frequent maintenance is necessary, and such a sealing member is not satisfactory as a sliding member.
That is, since a fuel to be used for combustion in the boiler for a thermal power generating plant and the like is coal, heavy oil or the like, corrosive gases such as oxides of sulfur and oxides of nitrogen are contained in the hot exhaust gas. Thus, dew drops are formed at a lower temperature portion of the air preheater, so that parts of the air preheater are likely to be corroded.
A proposed solution to the above problem utilizes a sealing member made of an inorganic glass or a wear resistive ceramic. This sealing member is preferably slid with, for example, a sealing bar, under pressure, and the sealing effect can be most effectively exhibited by so doing.
However, inventors' further examination revealed that certain problems remain unsolved. That is, since the heat exchanging efficiency of the air preheater is deteriorated when dust in the hot exhaust gas attach to the parts such as the heat elements of the air preheater, the air preheater is washed with water to remove the attached dust at periodically. Since a boiler for a thermal power generating plant or the like is continuously operated for an extended period of time and can only be stopped for a short period of time, the air preheater must be washed with water to remove the attached matter in the state that the air preheater is not fully cooled (i.e., at about 200.degree. C.).
Accordingly, the air preheater is thermally shocked due to a difference in temperature, for example 170.degree.-200.degree. C. Therefore, there is a problem in that when the sealing member employs alumina ceramic having extremely excellent wear resistance, the alumina ceramic will be broken by thermal shock.
Further, the rotary air preheater for combustion in a boiler to be used in a thermal power generating plant and the like is a large size apparatus in which the diameter of the rotor is 1-20 m, and parts used in the preheater are bulky. For example, a single sealing member having the size of 200.times.100.times.50 mm is used. Therefore, the size of the ceramic used for such a sealing member is naturally large. Thus, the sealing member is extremely fragile against thermal shock due to its shape and large size.